THE LIVER SECRETION OF BILE. 



619 



Respiration of these beings, which renders unnecessary any other complex apparatus for ela- 

 borating carbon from the system. 



b. On the other hand, among the Mollusca, the Liver attains a much greater development. 

 Instead of the follicles being prolonged into tubes (which is the usual form of the glandular 

 system in Insects), they are very much increased in number, and arranged on the sides of 

 canals or efferent ducts, which either separately pour their fluid into the intestine, or partially 

 unite with each other before doing-so. The Liver thus acquires a lobulated character, each 

 lobe consisting of a duct with its branching follicles; and the whole organ forms a consider- 

 able proportion of the mass of the viscera, and is evidently of great importance in the 

 economy of the animal. It is interesting to compare this complex structure with the very 

 simple condition presented by the Liver in insects ; and, when we keep in view the relative 

 amount of Respiration in the two groups of animals, we are at once struck with the fact, 

 that the development of the Liver bears an inverse proportion to the opportunity afforded 

 by the Respiratory organs for the aeration of the blood ; it being peculiarly extended, when 

 these, either from their small size, or from their employment in an aquatic medium, cannot 

 perform their function with great activity. This conclusion is confirmed in an interesting 

 manner by the fact, that the Crustacea, which have the general organization of Insects, but 

 which inhabit the water and breathe by gills instead of by a complex system of air-tubes, 



Fig. 223. 



Fig. 224. 



VAJ 



Lobule of Liver of Squilla Mantis ; exterior. 



Lobule of Liver of Squilla Mantis cut open. 



possess a Liver corresponding in form and in degree of development with that of the 

 Mollusca. 



c. In the Vertebrated Sub-kingdom, we may trace the operation of the same principle ; 

 but the internal structure of the Liver, in the adult condition at least, is less easily demon- 

 strated, than it is in the lower classes; owing to its increased complexity of structure, and 

 the closer union between its different parts. In Fishes and Reptiles, the Liver is of consider- 

 able size, and seems to perform a very important part in the decarbonization of the blood : 

 its form is adapted to that of the cavity in which it is lodged, sometimes one lobe only being 

 developed. In Birds, on the other hand, whose respiration is so much more active, it is much 

 smaller, but is placed on the median line, in conformity with the general symmetry of their 

 internal as well as external organs ( 40). In Mammalia, also, it is comparatively small ; 

 but, though reduced in proportional size, it is at the same time much more compact and 

 firm than in the lower Vertebrata. 



d. The Liver of Man is much less developed than that of many other Mammalia ; and 

 presents, as rudimentary indications, certain organs which are elsewhere fully developed. 

 The whole mass, which we are accustomed to describe as consisting of a right and left lobe, 

 does in reality form but one (there being no real division between its two portions), which 

 must be regarded as the Central lobe ; the Lobulus Spigelii is the rudiment of a second or 

 right lobe, and the Lobulus Caudatus is a Lobule developed from it. In the Carnivora and 

 Rodentia, which present the most complex form of Liver that we meet with among Mam- 

 malia, there are five distinct parts ; a central or principal lobe, corresponding with the 

 principal part of the liver of Man; a right lateral lobe with a lobular appendage, correspond- 

 ing to the Lobulus Spigelii and Lobulus Caudatus ; and a similar lobe and lobule on the left 

 side. 



e. The Gall-bladder is an appendage to the Liver, of which we find no traces in the 

 Invertebrata. It may be regarded as simply a dilatation of the efferent duct, more or less 

 prolonged from it, adapted to store up the hepatic-secretion against the time when it may be 

 required. In Fishes it frequently, but by no means constantly, presents itself; in Reptiles, 

 on the other hand, it invariably exists. In Birds it is occasionally absent, even in species 



